It is often necessary to combine the output from one or more light sources in an optical system. This may involve mixing light of the same wavelength from single or multiple sources or may involve mixing light of different wavelengths from multiple sources. A particularly important example is the production of white light by combining the output from discrete red (R), green (G) and blue (B) LED's. Efficient white light production from RGB LED arrays is a crucial enabling technology for the widespread use of LEDs for general lighting. RGB LED arrays are far more energy efficient than known LEDs based on phosphors. Improvements in the energy efficiency and effectiveness of mixing light, and in particular mixing light from LEDs, are therefore desirable to enable use of LED arrays in place of phosphor and related conversion systems to produce white light. Energy efficiency in lighting is usually measured in lumens provided per Watt of power used to supply these lumens.
An important factor is that the human eye is very sensitive to slight variations in colour. Therefore, one problem encountered in the mixing of light is that the required degree of colour uniformity, wherein colour variations in the resultant light are imperceptible to the human eye, cannot be achieved by current mixing methods without unacceptable losses.
Zhao, F. et al, “Optical Elements for Mixing Coloured LEDs to Create White Light”, conference paper 4776-35 at SPIE International Symposium on Optical Technology (SPIE's 47th Annual meeting, July 2002), published in Solid State Lighting II: Proceedings of SPIE volume 4776, pages 206–214, (2002) disclose an experimental investigation into the problems encountered and illustrates the deficiencies of the current state-of-the-art. Zhao, F. et al disclose using optically clear, acrylic light guides of square and circular cross section and of various lengths (76 mm, 152 mm, 305 mm) to mix red, green and blue light from an RGB LED array comprising 18 LED's.
The light guides were not found to significantly improve the beam uniformity in terms of illuminance and colour and there was no appreciable difference in the quality of the light produced by the different shaped light guides. The system energy efficiency deteriorated as a function of length as expected. The only improvement in illuminance and colour uniformity was achieved by diffusing the output ends of the light guides, which incurred further deterioration in efficiency.
Generation of white light is not the only application requiring efficient mixing of laser diode or LED output. Photo-curing of cements, such as dental cement requires a uniform illumination of the photosensitive material. Existing systems based on light guides produce uneven illumination giving unsatisfactory curing.
Hence, there is a need for a means of combining the output from one or more light sources to achieve output light with good uniformity of both colour and intensity across the output area. It is also desirable to have the ability to combine light from multiple sources without greatly increasing the angular spread of the input light and to make full use of the lumens supplied by the one or more sources.